Need Hale with prepaid plan. Help. Extend pla

Help please. I purchased a prepaid SIM card and plan last week in Berlin. I do not speak German. My account has run out after one week. I have access to the web and my account online via WiFi  but I do not know how to add more time. Each time it says download the app but the app does not want it. Please help since I have 3 more weeks of traveling around Europe and I'd like to know more time and money to the service.